Q: my ipad air storage is almost full. How do I purge it of the least significant items? Is there an app to purge duplicate photos and music?

First of all, have you checked where most of the storage is used for?

Check your storage on your iPhone, iPad, and iPod touch - Apple Support

You can see how much space each app is using via Settings > General > Storage & iCloud Usage > Manage Usage.

For your music and photos you will need to go through them yourself and delete anything that you don't want (for photos, if you've 'copied' photos into new albums directly on your iPad then they aren't duplicates, those albums only contain pointers to the original photos not separate copies - delete the original and you delete it from any album that you copied it into)

You can keep shared apps off your iPad by turning off Settings>iTunes & App Store>Automatic Downloads>apps.